1. Field of the Invention
The present invention relates to an error monitoring apparatus provided in a system for transmitting and receiving data of plural bits as a multi-value signal and, more particularly, to a multi-value signal monitor circuit which utilizes an analog-to-digital converter for regenerating the transmitted data as well as error detection data.
2. Description of the Related Art
Various systems have been previously proposed for transmitting data of plural bits after converting it into a multi-value or multi-level signal. For example, in a digital radio transmission system utilizing quadrature amplitude modulation (QAM), I-channel data and Q-channel data are respectively converted to multi-value signals, two carriers having a phase difference of 90 degrees therebetween are amplitude modulated by the multi-value signals and combined for transmission, the received signal is quadrature-detected by a regenerated carrier which is regenerated from the received signal, the I-channel and Q-channel multi-value signals thus obtained are converted into digital signals from the analog signal to obtain the original data.
In such a transmission system, if the radio link begins to fail due to fading, etc., the error rate for the received data becomes high and if the measured error rate exceeds a specified value, the transmission line or path is switched to a spare transmission line or path or an equalizer is reset to prevent divergence of the equalizer.
One method of measuring an error rate for received data involves transmitting a particular error detection pattern and checking the accuracy of the received pattern on the receive side. However, this method has the following disadvantages:
Transmission capacity is reduced by the size of the particular error detection pattern.
A complicated circuit is required for detecting the error detection pattern.
The error rate is usually small and a long time is taken for a cycle of error rate measurement or a large time constant for integration is required for an accurate measurement and, as a result, error rate threshold crossing detection is delayed.
Another method for measuring the error rate of the received data includes using a plurality of comparators for discriminating quantization levels and other levels of the received signal, and logic circuits for logically calculating outputs of the comparators. The number of times the received signal is displaced from the quantization level at the time of sampling the received multi-value signal is measured as a pseudo error rate and is used as the error rate. However, this method requires many comparators and the necessary logic circuits become more complicated as the number of bits of data to be transmitted increases and the number of quantization levels increases.
Another method of obtaining an error rate for the received data includes performing an analog-to-digital conversion at a normal timing and also at a timing slightly shifted from the normal timing. The number of times the shifted timing sample value is different from that at the normal timing is measured as a pseudo error rate and used as the error rate. However, this method has the following disadvantages:
Two analog-to-digital converters are required for each channel, one for normal timing conversion and one for the shifted timing conversion, so that the pseudo error rate can be detected, thereby the size and cost of the circuit is increased.
Since a rate of deterioration around a quantization level after a shift from the correct sample timing is different depending on the causes of line failure (for example, multi-path-fading by a reflected wave, attenuation of the signal by rain and failure of transmitting device), accurate correspondence to the error rate cannot be obtained.